sensors/daemonize.py

# #!/usr/bin/python

import fcntl
import os
import pwd
import grp
import sys
import signal
import resource
import logging
import atexit
from logging import handlers


class Daemonize(object):
    """ Daemonize object
    Object constructor expects three arguments:
    - app: contains the application name which will be sent to syslog.
    - pid: path to the pidfile.
    - action: your custom function which will be executed after daemonization.
    - keep_fds: optional list of fds which should not be closed.
    - auto_close_fds: optional parameter to not close opened fds.
    - privileged_action: action that will be executed before
                         drop privileges if user or
                         group parameter is provided.
                         If you want to transfer anything from privileged
                         action to action, such as opened privileged file
                         descriptor, you should return it from
                         privileged_action function and catch it inside action
                         function.
    - user: drop privileges to this user if provided.
    - group: drop privileges to this group if provided.
    - verbose: send debug messages to logger if provided.
    - logger: use this logger object instead of creating new one, if provided.
    """
    def __init__(self, app, pid, action, keep_fds=None, auto_close_fds=True,
                 privileged_action=None, user=None, group=None, verbose=False,
                 logger=None):
        self.app = app
        self.pid = pid
        self.action = action
        self.keep_fds = keep_fds or []
        self.privileged_action = privileged_action or (lambda: ())
        self.user = user
        self.group = group
        self.logger = logger
        self.verbose = verbose
        self.auto_close_fds = auto_close_fds

    def sigterm(self, signum, frame):
        """ sigterm method
        These actions will be done after SIGTERM.
        """
        self.logger.warn("Caught signal %s. Stopping daemon." % signum)
        os.remove(self.pid)
        sys.exit(0)

    def exit(self):
        """ exit method
        Cleanup pid file at exit.
        """
        self.logger.warn("Stopping daemon.")
        os.remove(self.pid)
        sys.exit(0)

    def start(self):
        """ start method
        Main daemonization process.
        """
        # If pidfile already exists, we should read pid from there;
        # to overwrite it, if locking
        # will fail, because locking attempt somehow purges the file contents.
        if os.path.isfile(self.pid):
            with open(self.pid, "r") as old_pidfile:
                old_pid = old_pidfile.read()
        # Create a lockfile so that only one instance of this daemon is
        # running at any time.
        try:
            lockfile = open(self.pid, "w")
        except IOError:
            print("Unable to create the pidfile.")
            sys.exit(1)
        try:
            # Try to get an exclusive lock on the file. This will fail if
            # another process has the file
            # locked.
            fcntl.flock(lockfile, fcntl.LOCK_EX | fcntl.LOCK_NB)
        except IOError:
            print("Unable to lock on the pidfile.")
            # We need to overwrite the pidfile if we got here.
            with open(self.pid, "w") as pidfile:
                pidfile.write(old_pid)
            sys.exit(1)

        # Fork, creating a new process for the child.
        process_id = os.fork()
        if process_id < 0:
            # Fork error. Exit badly.
            sys.exit(1)
        elif process_id != 0:
            # This is the parent process. Exit.
            sys.exit(0)
        # This is the child process. Continue.

        # Stop listening for signals that the parent process receives.
        # This is done by getting a new process id.
        # setpgrp() is an alternative to setsid().
        # setsid puts the process in a new parent group and detaches
        # its controlling terminal.
        process_id = os.setsid()
        if process_id == -1:
            # Uh oh, there was a problem.
            sys.exit(1)

        # Add lockfile to self.keep_fds.
        self.keep_fds.append(lockfile.fileno())

        # Close all file descriptors, except the ones mentioned in
        # self.keep_fds.
        devnull = "/dev/null"
        if hasattr(os, "devnull"):
            # Python has set os.devnull on this system, use it instead as it
            # might be different
            # than /dev/null.
            devnull = os.devnull

        if self.auto_close_fds:
            for fd in range(3, resource.getrlimit(resource.RLIMIT_NOFILE)[0]):
                if fd not in self.keep_fds:
                    try:
                        os.close(fd)
                    except OSError:
                        pass

        devnull_fd = os.open(devnull, os.O_RDWR)
        os.dup2(devnull_fd, 0)
        os.dup2(devnull_fd, 1)
        os.dup2(devnull_fd, 2)

        if self.logger is None:
            # Initialize logging.
            self.logger = logging.getLogger(self.app)
            self.logger.setLevel(logging.DEBUG)
            # Display log messages only on defined handlers.
            self.logger.propagate = False

            # Initialize syslog.
            # It will correctly work on OS X, Linux and FreeBSD.
            if sys.platform == "darwin":
                syslog_address = "/var/run/syslog"
            else:
                syslog_address = "/dev/log"

            # We will continue with syslog initialization only if
            # actually have such capabilities
            # on the machine we are running this.
            if os.path.isfile(syslog_address):
                syslog = handlers.SysLogHandler(syslog_address)
                if self.verbose:
                    syslog.setLevel(logging.DEBUG)
                else:
                    syslog.setLevel(logging.INFO)
                # Try to mimic to normal syslog messages.
                format_t = "%(asctime)s %(name)s: %(message)s"
                formatter = logging.Formatter(format_t,
                                              "%b %e %H:%M:%S")
                syslog.setFormatter(formatter)

                self.logger.addHandler(syslog)

        # Set umask to default to safe file permissions when running
        # as a root daemon. 027 is an
        # octal number which we are typing as 0o27 for Python3 compatibility.
        os.umask(0o27)

        # Change to a known directory. If this isn't done, starting a daemon
        # in a subdirectory that
        # needs to be deleted results in "directory busy" errors.
        os.chdir("/")

        # Execute privileged action
        privileged_action_result = self.privileged_action()
        if not privileged_action_result:
            privileged_action_result = []

        # Change gid
        if self.group:
            try:
                gid = grp.getgrnam(self.group).gr_gid
            except KeyError:
                self.logger.error("Group {0} not found".format(self.group))
                sys.exit(1)
            try:
                os.setgid(gid)
            except OSError:
                self.logger.error("Unable to change gid.")
                sys.exit(1)

        # Change uid
        if self.user:
            try:
                uid = pwd.getpwnam(self.user).pw_uid
            except KeyError:
                self.logger.error("User {0} not found.".format(self.user))
                sys.exit(1)
            try:
                os.setuid(uid)
            except OSError:
                self.logger.error("Unable to change uid.")
                sys.exit(1)

        try:
            lockfile.write("%s" % (os.getpid()))
            lockfile.flush()
        except IOError:
            self.logger.error("Unable to write pid to the pidfile.")
            print("Unable to write pid to the pidfile.")
            sys.exit(1)

        # Set custom action on SIGTERM.
        signal.signal(signal.SIGTERM, self.sigterm)
        atexit.register(self.exit)

        self.logger.warn("Starting daemon.")

        self.action(*privileged_action_result)